The present invention relates to an image sensor, more particularly to an image sensor which is constructed immune to noise originating from an alternating power supply.
LED (Light Emitting Diode) arrays, xenon lamps, fluorescent lamps and the like have been broadly used for lighting originals along the reading line in facsimile machines, copying machines and so forth. There are some difficulties in designing such machines as follows. Namely, while LED arrays emit little electromagnetic noise since they are powered by DC voltages, their emission intensities tend to vary along the length. FIG. 1 illustrates this situation. The abscissa represents the positions of 1728 diodes arranged in a line. Line 7 shows constant level of the brightness of an original from which light rays are reflected toward an image sensor. Curve 6 indicates the output signals of the image sensor at the positions corresponding to the respective diodes. As shown in the figure, the output levels of the diodes are different due to the disparity of the performances of the constituent LEDs.
On the other hand, xenon and fluorescent lamps can emit light rays whose intensities are equal along the length, but also produce adverse electromagnetic noise. FIG. 2 shows an exemplary output signal waveform of an image sensor employed in a conventional reading device in accordance with the prior art. As apparent from the figure, the output signal suffers disturbance, such as modulation, due to electromagnetic noise from an alternating power supplied to the lamp in accordance with the lighting repetition at the frequency of the alternating power supply.
Particularly, in case of facsimile machines in which an image sensor is located close to the light source, the image sensor, which has to control weak signals, is very sensitive to electromagnetic noise. It is desirable for stabilizing the output of the image sensor to insure against disparity of intensity over the length of the lamp. No lamp, however, is available at the present which can simultaneously overcome these two problems of the disparity of intensity and the electromagnetic noise. Examples of measure against the problems are the use of an electromagnetic shield and the provision of a shading compensation circuit, which in turn result in a complicated, large-sized and expensive structure.